Broadband amplifiers such as those used for radio frequency (RF) circuits are specified and designed to satisfy various criteria, primary among those are noise, distortion and gain. While every circuit adds some amount of noise and distortion to a signal, proper operation of a device often demands that these effects be limited to some acceptable level. Accordingly, designers specify an acceptable level of noise and distortion that the overall circuit can tolerate such that device operation is not degraded beyond some desired threshold level.
Signal distortion for broadband circuits such as broadband amplifiers is a function of and proportional to the overall total input signal level required to be accepted by a particular circuit; the greater the total overall input signal the greater will be the distortion level while operating at a particular point of the amplifier's operating curve. This becomes critical in circuits designed to accommodate a relatively broad frequency of input signals made up of, for example, a plurality of discrete carrier signals such as typically carried by a cable television or an off-the-air antenna system. Within the required bandpass of devices used by such systems (e.g., input or “RF” stage of a television receiver), a large number of receivable discrete carrier signals may be present spanning a range of signal strengths, i.e., some signals may be relatively weak and others quite strong, the latter possibly exceeding the maximum signal level handling capabilities of the input stage over which a desired degree of linearity may be achieved. That is, excessive total input signal power may result in exceeding the maximum dynamic range of the amplifier circuit over which an input-to-output transformation is substantially linear. A result of operating outside an acceptable linear portion of the amplifier circuit's operating curve is that excessive distortion may be introduced into the resultant output signal. While a particular device may ultimately select and cull from among many signals of a broadband signal a single signal for further processing, preceding broadband amplifier circuits must accommodate many adjacent signals constituting the broadband signal and having a total power that may be much greater than that of a single or some relatively small number of desired signal(s). To avoid introducing excessive distortion, these broadband amplifiers should be operated over an appropriate portion of their operating curves based on a worst case highest anticipated total signal power level rather than just that of the embedded desired signal(s). That is, the greater the total applied signal power, the more difficult it is to maintain input-to-output linearity necessary to achieve a desired maximum signal distortion level.
These broadband amplifiers must be operated along an appropriate portion of their operating curves for the total signal power level to be handled. In spite of large dynamic ranges of signal levels among the various channels, the system must provide for and accommodate the reception and detection of the weakest signals found among what may be many strong signals. These extraneous, non-selected signals may be termed “non-desired.”
In such an environment, certain assumptions must be made when specifying circuit design and operational criteria. Typical design practice takes the approach of accommodating the “worst case” situation under which a particular device is required to operate with a given performance criteria. Thus, for example, circuit design must provide a sufficiently low noise figure to accommodate the weakest signal level to be received but, at the same time, a distortion performance that will accommodate a high power level resulting from a large number of signals present at the input. For example, in a typical cable television system including 133 channels of programming transmitted on respective carrier signals, certain tuner arrangements may include some form of tracking filter that is dynamically adjusted to, partially or substantially, eliminate signals other than a selected signal or signals. However, in the absence of such a tracking filter (such as in tuners integrated on a single integrated microchip), the input circuitry must avoid signal distortion based on the summed power of all 133 channels even though all but one channel is to be rejected at some later (e.g., intermediate frequency or “IF”) stage. That is, the “non-desired” 132 channels contribute additional power to the input such that an input amplifier stage must be much more linear than if it needed to be designed to merely accommodate the power of the single selected channel/carrier signal.
Using standard circuit design criteria and methodology, all circuits are designed based on the maximum allowable noise figure that can still provide a suitable signal-to-noise ratio to provide specified signal processing for some minimum signal strength while, at the same time, these circuits must provide a threshold distortion capability in excess of the total power that might be presented in a worst case scenario of all channels having some maximum signal strength. While present at the input stage, some of the non-desired channel signals are removed at later, intermediate stages (e.g., IF) by appropriate filtering used to reject non-selected signals that thereby reduces the total power of the combined, IF signal. That is, intermediate bandpass filtering removes some of this power of the undesired signals while leaving the desired signal power substantially the same. Unfortunately, in a broadband system it is often difficult, if not impossible, to predict or have sufficient information about these “undesireds”, i.e., the non-selected signals, to determine an appropriate “over-design” margin. Absent this information, devices not having a tracking filter to reduce the power level of non-selected signals, design criteria is statically dictated by a possibly transient worst case total signal power situation.